Terminal Radar for Weather Avoidance -- Part 2

Last week I confessed that I am thrilled by technology. I like gadgets, especially those with buttons (I even like coats and shirts because they have buttons). So, here's a little more information about a marvelous piece of technology: ASR-9 terminal radar, and how it can provide you with valuable weather information.

First, let's talk a little more about buttons-I mean the ones on the ASR-9 weather control panel. Notice the split button labeled DISC/SUM as shown in Figure 1. This stands for discrete and sum.

When the SUM button is pushed (illuminated), the controller sees a combination (a sum) of VIP storm levels based on the (VIP) weather-level buttons selected. In other words, if the level-1 and level-3 buttons are pushed, then all weather levels from 1 to, but not including, 3 are combined and displayed as an area of low illumination (Figure 2). All weather levels of 3 and greater combine and display as an area of high illumination (Figure 2).

Here's another example. If the level-3 and level-5 weather-level buttons are pushed and SUM is selected, the weather appears as shown in Figure 3. Since level-3 is the first button pushed, the controller won't see a display of weather less than level-3. Weather from level-3 and level-4 is displayed as low illumination. Level-5 and above is displayed as high illumination. Of course, this assumes that there is level-5 weather available to display.

The DISC or discrete function allows the controller to see specific or individual bands of weather. It's a very useful function because the controller can visually identify as many as four specific levels of weather at one time. Here's how that works.

Let's assume a level-5 storm is present within the scanning radius of the radar. The DISC button is pushed (illuminated) along with the level-2 and level-4 weather-level buttons. With discrete selected, the controller can identify individual storm levels as shown in Figure 4.

The first band of weather is level-2 and is shown by a contour of low illumination. Inside the level-2 contour is an area with no weather return (no illumination). This, by default, must be a level-3 area. Toward the middle of the storm is a contour of high illumination. This is a level-4 return. Inside this is an area with no return (no illumination), which must be the level-5 portion of the storm. The assumption is that the weather levels increase toward the middle of the storm. A reasonable assumption to make, of course.

Just as an aside, in Part 1 of this series, I didn't show any portion of the DISC/SUM button illuminated. The assumption was that SUM was selected. There was no need to put any more "light" on the issue than was necessary. After all, these lights can initially be confusing, but they are pretty, aren't they? That's why an acquaintance of mine, despite his interest, never became a police officer. During his interview, when they asked him what he liked most about law enforcement, he replied, "The lights man - the lights, whooo, whooo, whooo!"

As you can see, the DISC function allows the controller to see as many as four specific storm levels at one time. SUM, however, allows the controller to see combinations of specific weather levels based on the weather-level buttons selected. How do these buttons help the ASR-9 controller?

Controllers and Weather Reporting

Some controllers like to watch the progression of storm levels. Observing the changing shapes of weather level contours allows them to identify a storm's growth or dissipation. Not that all controllers do this, of course. In fact, the amount of training a controller receives on displaying weather varies dramatically between ASR-9 facilities. That's why you, the pilot, need to know what to ask for and how to ask for it.

Most important, some controllers are reluctant to discuss weather with you. Remember, we live in a litigious society: one false move and lawyers are banging at your door. At least that's the way the government sees it. That's why controllers will gladly tell you what they see, and leave the interpretation up to you.

In fact, the controller's handbook instructs controllers to:

"Issue pertinent information on observed/reported weather or chaff areas. Provide radar navigational guidance and/or approve deviations around weather or chaff areas when requested by the pilot. Do not use the word 'turbulence' in describing radar-derived weather."

No problem here. As far as you're concerned, you can get all the radar weather you want and never have to mention the word weather (the "W" word). Simply request that the controller push buttons, tell you which lights are active and describe areas of illumination on his or her screen. Then ask for vectors as appropriate to avoid specific areas of illumination as we discussed in last week's article.

Avoidance Distance?

Last week I suggested having the controller select the level-1 and level-3 weather-level buttons as the least conservative button selection. I also mentioned that Captain Dave Gwinn (the radar expert) makes it a point to avoid level-2 returns when possible (me too). What gives?

Here's the skinny.

Level-3 and up are definitely the levels you want to avoid; they are the biggies (make sure you read these previous articles or none of this will make much sense: Operating Within the Envelope, Part-4 and Terminal Radar for Weather Avoidance, Part 1). As I previously discussed, a level-3 convective storm contains a 2% chance of severe turbulence and a 40% chance of moderate turbulence. These probabilities apply to any and all portions of the storm, not just the level-3 part. In other words, these turbulence probabilities apply to the level-1 and level-2 part of the storm.

Therefore, you need to avoid a level-3 convective storm. How far should you avoid it? The AIM (Aeronautical Information Manual) recommends that you avoid thunderstorms by 20 miles (we're assuming the level-3 return is, or will become, a thunderstorm). There's nothing new here. Just good common sense. There is, however, another recommendation you might consider.

A (very) major airline sets the following recommendations for its pilots in regards to storm avoidance. If a level-3 return exists and the tops of the storm are below 20,000 feet, avoid that storm by at least 10 miles. If the tops are above 20,000 feet, avoid the storm by at least 20 miles. If there are any indications that the storm may be severe, avoid the storm by at least 25 miles. What are the criteria identifying a severe storm?

Captain Dave Gwinn suggests that if the controller has at least four AVAIL (available) lights illuminated, then small airplane pilots should treat that storm as severe. That, my friend, is an excellent recommendation.

If you're keeping a 10 mile distance from a level-3 return and you discover (by asking the controller) that the storm has increased to a level-4 return, that's the time to ask for an additional 15 miles lateral distance. The controller would know this because another AVAIL light would automatically illuminate.

A More Conservative Selection

But what about having the controller push a more conservative selection of weather-level buttons? Perhaps the level-1 and level-2 buttons. Sure, if three or more AVAIL lights are shown and you request this button selection, you're being very conservative. The problem with this, as I see it, is that it limits your choices by restricting the information available to you.

For instance, suppose you're caught between that rock and a hard place and need to penetrate an area of weather. With the level-1 and level-2 buttons pushed, the controller sees the information shown in Figure 5. Let's say you chose to follow path A. Now let's have the controller select the level-1 and level-3 weather-level buttons as shown in Figure 6.

The path you chose (path A), puts you closer to the level-3 portion of the storm on your right. Path B would have been a better choice in terms of remaining farther away from a level-3 boundary, but you couldn't know this based on having the level-1 and level-2 buttons selected. That's why a level-1 and level-3 button selection is usually the best choice, in my opinion.

What You've Learned So Far

Let's summarize:

1. When using ASR-9 radar for storm avoidance, if only two AVAIL lights are shown, and convective weather is present, stay out of any level-2 returns, if possible.
2. If three AVAIL lights are shown, and convective weather is present, stay out of the entire storm by the following distances:
   1. tops of the storm below 20,000 feet - avoid that storm by at least 10 miles.
   2. tops of the storm above 20,000 feet - avoid the storm by at least 20 miles.

3. If the controller says that four AVAIL lights are shown, then assume the storm is severe for small airplanes. Avoid the storm by at least 25 miles.
4. If you are somehow forced into penetrating a storm (don't let this happen, of course), then give all level-3 areas as much lateral separation as possible.

Avoiding the higher VIP storm levels is wise indeed. After all, it's where the greatest concentration of water is. And more water means a greater potential for the rapid accumulation of ice.

In a Cessna 310, I once picked up 1/2 inch of clear ice in less than 30 seconds-absolutely no exaggeration here! I suspect this accumulation occurred in the higher VIP levels of the storm based on what others were saying over ATC's frequency. This happened at twilight and it was dark in the cloud, but I could see the ice because of the lightning. Isn't that exciting! A storm puts "b-b-b-ad things" on your airplane then lights it up so you and your passengers can see it. Mother Nature has a great sense of humor.

Temperature Inversions and Level-2

In Part 1 of this series, I mentioned that temperature inversions might cause a level-2 return. Yes, that's possible. That's why I've specifically mentioned convective weather in regards to avoidance criteria. How would you know if a temperature inversion is present? The clouds are your clue.

Stratus clouds normally indicate that a temperature inversion is present. After all, cloud formation releases latent heat; the released heat causes parcels of air to rise; rising air can lead to cumulus cloud formation. The reason stratus clouds are flat on top is because they can't rise. Warmer air aloft stymies their vertical growth.

How would you know you're flying in stratus-type clouds? The air is usually smoother than in cumulus clouds, the water droplets are smaller, there's little or no vertical development and the air is generally stable.

Therefore, if you're flying in stratus clouds and the controller has two AVAIL lights shown, these lights may very well be the result of a temperature inversion. Temperature inversions can reflect radar energy back to the radar antenna, as if displaying something solid like water. No biggie here. Keep in mind, however, that certain parts of the country, like Florida, may have stratus clouds that produce a level-3 return. This doesn't make these clouds hazardous from a turbulence perspective. They just contain a lot of water. It's not a problem if temperatures are above freezing, since there's no ice potential here. If temperatures are below freezing, that's a potential problem (and an entirely new article).

You may be wondering why I told you about the DISC/SUM buttons and didn't mention using them as part of the pilot-controller discussion. We'll, if you're talking with an ASR-9 controller and storm weather is present, you'll understand what he means when he says that he sees an area of no illumination between a low and a high illumination interval. It means the controller has the DISC mode active while the level-1 and level-3 VIP buttons are pushed. If you didn't know anything about the DISC/SUM function, you'd probably be confused.

One last thing. It's important to keep in mind that ASR-9 radar is only one more tool you can use to make weather decisions. It's not, by any means, the only one. In my opinion, there are many additional information sources you need to check. In a future article we'll talk about these sources.

Till then, fly safe.

Note: Special thanks to my friends Rick Crose and Dave Gwinn for their technical assistance.

For more information on this subject, see "Weather Avoidance: Using all the ATC tools available to you."